Human tumor-inducing herpesviruses, Kaposi's sarcoma-associated herpesvirus (KSHV) and Epstein Barr virus (EBV), are frequently associated with malignancies in human immunodeficiency virus-1 (HIV-1) infected patients. While EBV latent genes are essential as primary contributors to oncogenesis, several KSHV lytic genes either behave as oncogenes or at least have growth-promoting properties, suggesting that KSHV lytic replication may contribute to the disease progression. Furthermore, KSHV lytic replication has been postulated to be an important exogenous factor that influences the severity and rate of disease progression in HIV-1 infected patients, reducing survival and increasing the risk of transmission. The goal of this study is to better understand how KSHV lytic replication contributes to the development of pathogenic events and influences the life cycle of co-existing viruses, EBV and HIV-1, with a specific focus on the KSHV Transcription and Replication Activator (RTA) gene. Previous work has defined that KSHV RTA plays a central role in switching the viral life cycle from latency to lytic replication. In addition, KSHV RTA mimics the Notch signal transduction pathway to induce viral and cellular gene expression. Specifically, the induction of CD21 surface antigen and soluble CD23 protein by KSHV RTA/Notch signal transduction effectively facilitates EBV and HIV-1 infection, respectively. Based on these preliminary results, we hypothesize that the molecular mimicry of cellular Notch signal transduction by KSHV RTA plays an important role not only in inducing cellular and viral gene expression but also in providing a favorable milieu for co-existing viruses, EBV and HIV-1. Our biochemical and genetic studies in Aim 1 will define in greater detail the mechanisms used by RTA and cellular Notch signal transduction to induce viral gene expression. To define the contributory role of RTA, we will investigate in Aim 2.1 how RTA expression influences EBV and HIV-1 life cycle in culture. To further investigate the effect of RTA/Notch signal transduction on the disease induction and life cycle of coexisting viruses in vivo, we will use the well-established rhesus monkey animal model that is a natural host of rhesus monkey rhadinovirus (RRV), the closest relative to KSHV, and develops acquired immunodeficiency syndrome (AIDS) upon infection with simian immunodeficiency virus (SIV), the closest relative to HIV-1. In Aim 2.2, we will investigate whether the experimental infection of rhesus monkeys with recombinant RRV facilitates pathogenic events in the presence or absence of SIV infection. Utilizing these well-established in vitro and in vivo experimental conditions, the proposed study will detail the roles of KSHV and RRV RTA in the regulation of viral and cellular gene expression and in the development of pathogenesis.